This invention relates to a floor system for seismic isolation which exhibits a proper seismic isolation function against an external vibration or force, such as an earthquake.
For example, electronic computers and generators for emergency, and hazardous materials, such as dynamite and chemicals, are readily sensitive to an adverse influence by the above-mentioned. Such apparatus, equipments, or casings for holding hazardous materials must be effectively protected.
In order to protect such vibration-sensitive apparatus and hazardous materials against an external vibration, two countermeasures have conventionally been adopted as set forth in more detail below:
The first countermeasure is to properly design such apparatus per se or the equipment for holding such hazardous materials so as to enhance their rigidity and to increase their natural frequency so as to avoid a possible resonance to the frequency of the external vibration. However, the first countermeasure involves highly expensive apparatuses or equipments and is not practical.
The second countermeasure is to provide a seismic isolation function to a floor on which various vibration-sensitive apparatus are installed. A number of elastic seismic elements are located, between a floor member and a foundation elastically supporting the floor member. A system for seismic isolation including the elastic members, floor member, and apparatuses or equipments which are placed on the floor member, is set to a very small natural frequency, thereby avoiding the resonance of the system against the external vibration. Since, according to the second countermeasure, the external vibration can be absorbed by the elastic members, only a damped vibration is, in reality, transmitted to the apparatus or the equipment on the floor member. It is, therefore, not necessary to provide an adequate rigidity to the apparatuses or equipments. As a result, the cost of the apparatus equipments is not higher than in the case of the first countermeasure.
U.S. Pat. No. 4,371,143 discloses a floor system for seismic isolation as an example of the embodiment of the second countermeasure. The known floor system includes a number of apparatus which exhibit a seismic isolation function and are arranged between the floor member and the foundation. Stated in more detail, the seismic isolation apparatus comprises a plate-like base disposed on the foundation, a plate-like carrier so disposed on the base as to be slidably moved in a horizontal direction, and coupled to the lower surface of the floor member, and anti-seismic holders for elastically holding the carrier against the base. Here, the anti-seismic holder comprises an elastic element which encloses the outer peripheral portion of the carrier and adapted to tighten the carrier, from outside, with a predetermined force, and stopper elements secured to the base to permit them to be regulated against movement in a direction in which the elastic element is contracted.
In consequence, in the normal state, the carrier is held on the base in a predetermined position. Stated in more detail, the elastic element comprises four tension coil springs arranged along the respective sides of the carrier and four connector members connecting the respective tension coil springs together.
According to the floor system for seismic isolation, if a vibration force exceeding a tension force initially given to the tension coil springs is transmitted to the seismic isolation apparatus due to, for example, the occurrence of an earthquake, the carrier and, thus the floor member, is moved in the horizontal direction against an urging force of the tension coil springs. That is, the vibration energy of the earthquake is stored in the tension coil springs, thereby damping a vibration force actually transmitted to the apparatus on the floor member.
According to the above-mentioned floor system, however, if a breakage occurs at one location of the elastic element surrounding the carrier at one tension coil spring, the elastic element completely fails to perform its own function. It is therefore impossible to exhibit the above-mentioned seismic isolation function.
There is also a disadvantage that if a rotational force acts, in a horizontal direction, upon the floor member by a vibration resulting from an earthquake, it is not possible to effectively decrease the rotational movement of the floor member. Now suppose that a number of seismic isolation apparatus are arranged, in a matrix array, between the foundation and the floor member. In this case, only those seismic isolation apparatus located in closest proximity to the outer periphery of the floor member effectively serve to suppress the rotational movement of the floor member. Stated in more detail, a force acting upon the respective seismic isolation apparatus when the floor member tends to rotate around a center point corresponds to a turning effort represented by a product of the rotational force and a distance from the center point to the seismic isolation apparatus. Thus, tension springs of the seismic isolation apparatus arranged in closest proximity to the outer periphery of the floor member most effectively serve to suppress the rotational movement of the floor member. Moreover, among the four tension coil springs of the seismic isolation apparatus, only one tension coil spring closest to the outer periphery of the floor member most effectively functions to suppress the rotational movement of the floor member. Even if a number of seismic isolation apparatuses are provided below the floor member, it is not possible to effectively suppress the rotational movement of the floor member and thus to obtain a greater seismic isolation effect.
Since the four tension coil springs are so linked as to surround the carrier, the known seismic isolation apparatus becomes bulkier and complex due to the presence of the four tension coil springs.